1. Field of the Invention
The present invention relates to suspension systems for off-road equipment, such as agricultural tractors, and more particularly to such suspension systems that are dynamically adjustable in response to load changes and operation of the vehicle.
2. Description of the Related Art
Off-highway equipment, such as construction and agricultural vehicles, can carry widely varying loads. When a relatively heavy load is applied to the equipment, the vehicle body is forced downward with respect to the axles supporting the wheels of the vehicle. This results in compression of the suspension which can adversely affect the maneuverability of the vehicle. On the other hand, if the suspension is configured for very heavy loads, the vehicle will have an undesirable ride under light load conditions.
As a result, many vehicles have automatic load leveling systems which employ separate hydraulic cylinders between the axle and the frame on opposite sides of the vehicle. When a heavy load is applied to the frame, the drop of the frame is sensed and additional hydraulic fluid is applied to the cylinders to raise the frame to the desired distance from the axle. Thereafter, when that load is removed from the vehicle and the frame rises significantly above the axle, hydraulic fluid is drained from the cylinders to lower the frame with respect to the axle. This type of automatic hydraulic load leveling system ensures that the frame and axle are maintained at the desired separation regardless load applied to the vehicle.
The suspension undergoes another condition when the vehicle makes a relatively tight turn. At that time, the upper part of the vehicle body tips outward causing a roll effect that compresses the suspension cylinder on the outside of the turn and extends the cylinder on the inside of the turn. A greater force is applied to the outside cylinder than is encountered by the inside cylinder. In one configuration of the suspension, fluid in the outside cylinder is forced from the head chamber into the rod chamber. That flow is restricted because the volume of the rod chamber expands less that the contraction of the head chamber due the presence of the piston rod in the rod chamber. The restriction provides resistance to the roll motion. The amount of resistance determined by fixed size of the cylinders, which is selected based on the characteristics of the vehicle.
The fixed roll resistance, while being acceptable for many vehicles, is not optimal for vehicles in which the roll forces vary significantly under different operating conditions. For example, an agricultural vehicle, that is used to spray pesticides and herbicides on crop fields, has spray heads mounted on retractable booms that project laterally from each side of the vehicle body. The roll forces produced when the booms are extended for spraying are significantly greater than the roll forced when the booms are retracted against the body for travel to and from the field. Therefore, suspension cylinders selected to counter roll when the booms are retracted may not adequately counter the roll when the booms are extended. Selecting the suspension cylinders to counter roll of the vehicle with extended booms provides poor ride characteristics during both turning and traveling straight ahead.